Mechanism of CH 2 + Transfer from Distonic Ions X ‐ CH 2 + (X = CH 2 O, CH 2 CH 2 ) to π‐ and n‐Electron Bases in the Gas Phase. A Fourier Transform Ion Cyclotron Resonance (FTICR) Study Supplemented by ab initio MO Calculations

The reactions of acetonitrile, propyne, acetylene, trimethyl‐silylacetylene, and tetramethylsilane, with distonic ions CH 2 XCH 2 + [X = CH 2 ( 2 ), O ( 3 )] are studied in the gas phase using Fourier Transform Ion Cyclotron Resonance (FTICR) mass spectrometry. In line with previous studies, CH 2 + is transferred to the electron Ione‐pair of the nitrogen atom of CH 3 CN to generate CH 3 CN–CH 2 + ( 4 ); upon collisional excitation, this ion undergoes competitive losses of H * and CH * . While both neutrals originate from the “methylene” unit of 4′ , detailed studies employing labeled substrates and using various types of collision experiments reveal an intriguing dissociation pattern in that the dissociations are preceded by two intramolecular hydrogen migrations giving rise to CH 3 C(H) = NCH * + ( 6 ) and CH 3 C = N(H)CH *+ ( 7 ). While 6 serves as intermediate en route to loss of H * from the “CH” moiety, 7 is the actual precursor to generate, by loss of CH * , protonated acetonitrile, CH 3 CNH + ( 12 ) (Scheme 5). In addition, 12 is formed by bimolecular proton transfer. In this reaction, translationally excited CX 3 CN – CY 2 + * (X, Y = H, D) transfers X + ‐ to neutral CX 3 CN to generate CX 3 CNX + (Scheme 4). The bimolecular proton transfer as well as the intramolecular isomerizations of 4 to 6 and 7 are subject to very large kinetic isotope effects. In the transfer of CH 2+ to CH 3 C = CH two products are formed [i.e. H 3 C ‐ C ≡ C ‐ CH 3+ ( 16 ) and CH 2 C = CHCH 3+ ( l7 ) presumably via intermediate 18 (Scheme 6)]; the latter is formed by addition of CH 2+ to the less hindered carbon atom of HC ≡ CCH 3 reflecting the higher stability of the so‐formed intermediate compared with addition to C‐2, Reactions of 2 and 3 with HC ≡ CH do not result in the formation of a detectable CH 2+ ‐transfer product. When using CH 2 CH 2 CH 2+ ( 2 ) the reaction is prohibited by the endothermicity to generate the initial complex (structurally related to 18 ). On the other hand, when CH 2 OCH 2+ ( 3 ) is employed, the intermediate of CH 2+ transfer is formed with sufficient energy to split off a hydrogen atom. Preliminary experiments with silicon‐containing molecules, like Si(CH 3 ) 4 or HC ≡ CSi(CH 3 ) 3 , demonstrate that the favored processes of these neutrals with 2 and 3 are due to charge transfer (in the form of an electron or an anion like CH ‐ 3 or C 2 H ‐ ) from the silicon‐containing molecule to the distonic ions. The experimental results obtained for the CH 3 CN/CH 2+ system are supported by ab initio MO calculations (3‐21G/3‐21G + ZPVE).